The stock market had one of its worst days in months today after some disappointing news about manufacturing. The Dow Jones Industrial Average fell more than 300 points, about two percent. The other major indexes were down even more.

NPR's Jim Zarroli tells us investors are reacting to new concern about the health of the global economy.

JIM ZARROLI, BYLINE: Last year was a terrific year for the stock market, and so a lot of people on Wall Street were expecting the rally to keep going. Instead the S&P 500 index is down more than five percent this year. The big decline came after the Institute for Supply Management reported that manufacturing activity dropped sharply in January.

JACK ABLIN: We had some disappointing earnings results from the automakers here at home. We also had Chinese manufacturing data came in a little bit less than expected.

ZARROLI: Ablin says the decline in manufacturing in China is seen as something of a barometer for the health of the global economy. Ablin says the drop in economic activity has been abrupt. The U.S. economy grew by a healthy 3.2 percent during the last three months of 2013. And Ablin says the suddenness of this decline suggests that weather may have played a part.

ABLIN: I'll tell you, if you're looking to buy a car, I don't think you're going to want to tiptoe around in some new car lot or test drive the car when it's, you know, minus 10 degrees out.

ZARROLI: The decline in the data raises new questions about what the Federal Reserve is likely to do. Fed policymakers have been easing up on some of the economic stimulus measures they're taking because they thought the economy was doing a lot better. But the most recent jobs report was a lot weaker than expected. That report, coupled with today's manufacturing data, suggests that the rebound in the economy may not be as strong as Fed officials believed.